Cells are constantly exposed to a variety of environmental agents some of which induce DNA damage i.e. genotoxic stress. Cellular responses to genotoxic stress are complex and therefore, more studies are needed to better understand the mechanisms that control such responses. Here we propose to characterize a novel gene, which we have named PIQ (p53-regulated IQ protein) that codes for an IQ motif protein. IQ motifs promote protein interactions with calmodulin (CaM). CaM is a calcium-binding protein that controls several important signaling events. Evidence presented here show that PIQ does indeed bind to CaM. Furthermore, PIQ mRNA is overexpressed in primary colon tumors when compared with matching normal tissues and that PIQ is down-regulated in response to genotoxic stress and p53 activation. PUMA is a key mediator of p53- dependent genotoxic stress-induced apoptosis and our results indicate that PIQ down-regulates PUMA promoter expression. Importantly, PUMA promoter harbors regulatory elements that are predicted to be regulated by CaM-dependent signaling pathways. We, therefore, propose that in unstressed cells, PIQ may negatively regulate PUMA expression by interfering with CaM-dependent signaling events and PIQ downregulation following genotoxic stress serves to promote PUMA upregulation partly via CaM-dependent pathways. Our overall hypothesis is that PIQ is a negative modulator of apoptosis and that higher levels of PIQ expression noted in colon cancer may alter the threshold of apoptosis and consequently contribute in part to colon tumorigenesis. Three specific aims are proposed here to further characterize PIQ in order to elucidate its role in cellular response to genotoxic stress in general and in the development and/or progression of colon cancer in particular. Specific Aim 1 is to investigate the role of PIQ during p53 and genotoxic stress-induced apoptosis. Specific Aim 2 is to investigate the mechanism by which PIQ negatively regulates PUMA promoter expression. Specific Aim 3 is to analyze PIQ expression at the mRNA and protein levels in a larger pool of primary colon tumors and their matching normal tissues and to evaluate a relationship between tumor PIQ-status and the clinicopathologic features. [unreadable] [unreadable]